<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .screen {
            width: 200px;
            height: 100px;
            margin: 0 auto;
            background-color: grey;
            color: white;
            font-size: 30px;
            line-height: 100px;
            text-align: center;
            border-radius: 10px;
        }
        .add-btn {
            margin: 20px auto;
            width: 200px;
            height: 100px;
            font-size: 30px;
            line-height: 100px;
            display: block;
        }
    </style>
</head>
<body>
    <div class="screen">0</div>
    <button class="add-btn">+</button>

    <script>
        let screen = document.querySelector('.screen');
        let addBtn = document.querySelector('.add-btn');
        addBtn.onclick = function() {
            let number = screen.innerHTML;
            number = parseInt(number);
            number += 1;
            screen.innerHTML = number;
        }
    </script>

</body>
</html>